I don't think there's one answer to that, unfortunately. It depends on what you're buying; whether that product is in a development stage; whether it's a mature product that you're buying off the shelf, in essence a commodity; and that it can shift during a very long program.
For instance, if one were going to buy some helicopters and they were in production, then you might say the risk should be placed on the contractor who has a production line and proven design. However, if you then say you would like your helicopter to have five rotors instead of four, and to have more gas tanks and carry more equipment and fly farther, then you have changed the structure of your requirement in such a way that you're now into a development cost. At that point I think the government should be in a position where they are taking on more risk.